V. Report on Hygrometric Methods; First Part, including the Saturation Method 

 and the Cfiemical Method, and Dew-point Instruments. 



By W. N. SHAW, M.A. 

 Communicated by R. H. SCOTT, F.R.S., Secretary of the Meteorological Council. 



Received January 17, Bead January 26, 1888. 



[PLATE 5.] 



IN August, 1879, at the request of the Meteorological Council, I undertook an 

 experimental comparison of the various methods of determining the hygrometric 

 state of the air, with the following instruction, " The chemical method to be 

 employed, and with it to be compared the dry -and- wet-bulb hygrometer, REGNAULT'S, 

 DINES'S, ALLUARD'S, and the hair hygrometer." Since that time I have devoted to 

 the subject the time that was at my disposal, and I now beg leave to lay before the 

 Council a statement of the experiments I have made and the results I have arrived 

 at with respect to the chemical method and the dew-point instruments. 



The arrangement of the experiments was left to my discretion : I had, therefore, 

 first to practise myself in the use of the different methods in order to arrange in 

 some sort of order the means by which the various inquiries should be undertaken ; 

 and, further, it was necessary for me to know and to carefully consider the very 

 numerous contributions made by other observers to the discussion of hygrometric 

 methods, so that I might be able to distinguish between those points which had been 

 satisfactorily and permanently settled and those upon which further experimental 

 investigation might throw additional light. The distinction proved to be not a very 

 easy one to draw, and I shall therefore append to this report a summary of the work 

 done iu the subject since the time of DANIELL. I made a summary of this kind for 

 my own use at the outset, but since then many important memoirs have been 

 published, chiefly on the Continent, which bear particularly upon the question of the 

 trustworthiness of observations with the wet and dry bulb. I have taken account of 

 those memoirs in the summary which I now offer. It will, I believe, be found to 

 justify the following general conclusions : 



1. There is no hygrometric method of which it has been proved that an observer 

 following out definite written instructions with due care and skill can obtain measure- 

 ments of vapour pressure which are accurate to within 1 per cent. The accuracy 

 claimed by RENAULT for the chemical method is only " about a fiftieth," and to 



MDOCCLXXXV1II. A. L 2.7.88 



